Precoding a transmission from an antenna array involves applying a net of complex weights to the signals that are to be transmitted from the array's antenna elements, so as to independently control the signals' phase and/or amplitude. This set of complex weights is referred to as a “precoder”. The transmitting node conventionally chooses the precoder to match the current channel conditions on the link to the receiving node, with the aim of maximizing the link capacity or quality. If multiple data streams are simultaneously transmitted from the array's antenna elements using spatial multiplexing, the transmitting node also typically chooses the preceder with the aim of orthogonalizing the channel and reducing inter-stream interference at the receiving node.
In closed-loop operation, the transmitting node selects the precoder based on channel state information (CSI) fed back from the receiving node that characterizes the current channel conditions. The transmitting node in this regard transmits a reference signal from each antenna element to the receiving node, and the receiving node sends back CSI based on measurement of those reference signals. Transmission of the reference signals and feedback of the CSI contribute significant overhead to preceding schemes. For example, these reference signals and CSI feedback consume a significant amount of transmission resources, such as time-frequency resource elements in Long Term Evolution (LTE) embodiments.
Known approaches reduce overhead attributable to reference signal transmission by dedicating a reference signal for CSI measurement. LTE Release 10, for example, introduces a CSI Reference Signal (CSI-RS) specifically designed for CSI measurement. Unlike the cell-specific common reference signal (CRS) in previous LTE release, the CSI-RS is not used for demodulation of user data and is not precoded. Because the density requirements for data demodulation are not as stringent for CSI measurement, the CSI-RS can be relatively sparse in time and frequency, thereby reducing the number of transmission resources required for transmitting the CSI-RS.
Known approaches reduce overhead attributable to CSI feedback by limiting the usable precoders to a fixed set of precoders, i.e., a codebook. Each precoder in the codebook is assigned a unique index that is known to both the transmitting node and the receiving node. The receiving node determines the “best” precoder from the codebook, and feeds back the index of that precoder (often referred to as a “preceding matrix indicator”, PMI) to the transmitting node as a recommendation (which the transmitting node may or may not follow). Feeding back only an index, in conjunction with other CSI such as the recommended number of data streams (i.e., transmission rank) for spatial multiplexing, reduces the number of transmission resources required for transporting that CSI. This approach therefore reduces CSI feedback overhead considerably as compared to explicitly feeding back complex valued elements of a measured effective channel.
With the expected introduction in LTE Release 13 of a limited number of predetermined codebooks adapted for two-dimensional antenna arrays, there comes a desire to implement efficient adaptability of such codebooks. In particular, a codebook adaptation should preferably avoid one or more of: wasting processing resources at a transmitting end; wasting processing resources at a receiving end; requiring an excessive amount of collected (measured) data to provide a useful result; generating added signaling overhead.